Lake Rotoiti experienced significant degradation in water quality from the 1960s to the early 2000s due to nutrient-laden waters flowing from Lake Rotorua via the Ōhau Channel. To address this, an inlake diversion wall was constructed in 2007/2008 to block the influx of these nutrient-rich waters. However, during this period, water quality in Lake Rotorua also improved due to enhanced catchment management practices and alum dosing. Consequently, there is now a question of whether and to what extent the diversion wall is still necessary to prevent water quality deterioration in Lake Rotoiti, at its current Trophic Level Index (TLI) and target TLI of 3.5. This report presents the findings and implications of a comprehensive study commissioned by the Bay of Plenty Regional Council to evaluate the impact of the Ōhau Channel diversion wall on the water quality of Lake Rotoiti, New Zealand. The study, guided by specific research questions outlined in the “Terms of Reference: Ōhau wall: 7-year review”, aimed to assess the effectiveness of the wall in improving water quality, its influence on achieving TLI targets, and its impact on the quality of the Kaituna River. Additionally, the study investigated the potential consequences of removing the diversion wall, considering the altered water residence time and its effects on bottom water dissolved oxygen (DO) levels. An assessment of the holes in the diversion wall was also carried out to understand the amount of water leaking through the wall into Lake Rotoiti. To address the above research questions, this study consisted of three main components: 1) analysis of observed changes in water quality, 2) lake system modelling, and 3) assessment of holes in the diversion wall. Data analysis involved statistical analyses to determine differences in water quality pre- and post-wall construction. Lake system modelling included configuring a 3-D hydrodynamiconly model to assess for variation in hydrodynamics, and a coupled hydrodynamic-ecological model to assess for variation TLI with and without the wall using scenario simulations over a prolonged period (i.e., 19 years for the hydrodynamic model; 8 years for the hydrodynamic-ecological model). Assessment of wall holes involved data analysis and hydrodynamic model scenario testing to evaluate their impact on Lake Rotoiti. The data analysis of existing water quality data for Lake Rotoiti, employing a three-step approach (in increasing levels of complexity and inference), revealed nuanced insights into the impacts of the Ōhau Channel diversion wall. While descriptive statistics (i.e., approach 1) indicated overall improvement in water quality post-wall construction in Lake Rotoiti, correlation analysis (i.e., approach 2) between water quality in Lake Rotorua and Rotoiti suggests limited statistical significance in observed differences, except for Secchi depth, indicating altered connectivity between Lake Rotorua and Rotoiti. An intervention analysis (i.e., approach 3), employing interrupted time series analysis, detected significant step changes in total nitrogen (TN), Secchi depth, and TLI post-wall construction, suggesting immediate positive effects on lake water quality persisting throughout the study period. The estimated improvements due to the diversion wall were a decrease in TN by 203 mg m-3, an increase in Secchi depth by 2.5 m, and a decrease in TLI by 0.59 units. The data analysis supports the long-held observations of the Lake Rotoiti community, who have noticed a visible improvement in water quality immediately following the construction of the diversion wall. The data analysis revealed that the diversion wall has had no significant impact on bottom water DO demand in Lake Rotoiti, as measured by volumetric hypolimnetic oxygen demand, which remained largely unchanged. Any yearto-year variations are likely attributed to changes in prevailing in-lake and meteorological conditions. Hydrodynamic model simulations revealed that the wall was effective in reducing the accumulation of Ōhau Channel inflow in the system, increasing the fraction of Ōhau Channel inflow short-circuited down the Kaituna River and, consequently, increasing the residence time of Lake Rotoiti. In terms of accumulation of Ōhau Channel-derived water in Lake Rotoiti, the wall resulted in a substantial reduction as evidenced by an annual cumulative contribution of Ōhau Channel water to Lake Rotoiti of 22.0% without a wall in place, but 0.3% with a wall in place. Concerning the fraction of Ōhau Channel-derived water short-circuited down the Kaituna River, the wall resulted in a substantial increase, as evidenced by the proportion of Ōhau Channel water being diverted down the Kaituna River being 55.2% without a wall in place, but 99.3% with a wall in place. Regarding the residence time in Lake Rotoiti, the wall resulted in a substantial increase, i.e., a factor of 3.3, as evidenced by a residence time of 8.2 years without a wall in place, but 26.9 years with a wall in place. Coupled hydrodynamic-ecological model simulations revealed the removal of the wall lead to a small deterioration of water quality in Lake Rotoiti, which could be mitigated with improved water quality in the Ōhau Channel inflow. The various scenario testing revealed that removal of the wall alongside a maximum TLI in Lake Rotorua of ~4.3 would likely be required to maintain the current TLI in Lake Rotoiti; however, removal of the wall alongside a maximum TLI in Lake Rotorua of ~4.0 would be required to maintain the current TLI and each of Trophic Level nitrogen, Trophic Level phosphorus, and Trophic Level chlorophyll a. Further, removal of the wall alongside even the most ambitious reduction in maximum TLI in Lake Rotorua (i.e., 3.8) would require additional water quality management in Lake Rotoiti to achieve the target TLI in Lake Rotoiti of 3.5. The analysis of available data on the holes in the diversion wall indicates that despite a relatively small number of estimated holes in the wall, the leakage of Ōhau Channel water into Lake Rotoiti is appreciable; i.e., assuming a total of 100 holes along the length of the diversion wall, approximately 3.3% of the Ōhau Channel water was estimated to leak through the holes into Lake Rotoiti. This percentage increased to 9.8% of the Ōhau Channel water when the total number of holes was assumed to be 300. These discharge rates compare well with the hydrodynamic modelled output, where a single 1 × 100-meter hole (the finest scale hole possible within our model grid) in the Ōhau Channel diversion wall was shown to reduce the effectiveness of the wall in preventing the accumulation of Ōhau-derived water within the lake by 29.3-55.0%. However, the study's estimates are conservative and limited by the model resolution. Given uncertainties in the number of holes, the focus was on estimating water discharge rather than assessing nutrient load or water quality impacts. Overall, the evaluation of water quality in Lake Rotoiti, including a systematic analysis of long-term datasets and detailed 3-D hydrodynamic-ecological modelling, shows that improvements in water quality in the lake were evident following the construction of the diversion wall. This study shows that the Ōhau Channel diversion wall was critical in preventing the degradation of Lake Rotoiti, but meeting the TLI target in the lake will be challenging if the removal of the wall occurs, even if TLI targets in Lake Rotorua are met.